Sulfide-based solid electrolyte and preparation method thereof

ABSTRACT

A sulfide-based solid electrolyte contains a nickel (Ni) element and a halogen element. For example, a sulfide-based solid electrolyte can include, with respect to 100 parts by mole of a mixture of lithium sulfide (Li 2 S) and diphosphorus pentasulfide (P 2 S 5 ), 5 parts by mole to 20 parts by mole of nickel sulfide (Ni 3 S 2 ), and 5 parts by mole to 40 parts by mole of lithium halide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. § 119(a) the benefit of KoreanPatent Application No. 10-2016-0156040 filed on Nov. 22, 2016, theentire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a sulfide-based solid electrolyte anda preparation method thereof.

BACKGROUND

Today, secondary batteries have been widely used from large devices suchas a vehicle and a power storage system to small devices such as amobile phone, a camcorder, and a laptop.

As the secondary battery is widely used and applied, the demand forimproved safety and high performance of the battery has been increased.

A lithium secondary battery which is one of the secondary batteries hasan advantage that energy density is higher and a capacity per unit areais larger than a nickel-manganese battery or a nickel-cadmium battery.

However, most of the electrolytes used in the lithium secondarybatteries in the related art are liquid electrolytes such as organicsolvents. Accordingly, safety problems such as leakage of electrolytesand the risk of fire resulting therefrom have been constantly raised.

As a result, recently, to increase safety, an interest inall-solid-state batteries using solid electrolytes rather than liquidelectrolytes as the electrolytes has been increased.

The solid electrolyte has higher safety than the liquid electrolyte dueto a non-combustible or flame-retardant property.

The solid electrolytes are divided into an oxide-based electrolyte and asulfide-based electrolyte. The sulfide-based solid electrolyte has highlithium-ionic conductivity compared to the oxide-based solid electrolyteand is stable in a wide voltage range and thus the sulfide-based solidelectrolyte is frequently used.

In Mizuno et al., high lithium ion conducting glass-ceramics in thesystem Li₂S—P₂S₅ , Solid State Ionics, 177(2006), 2721-2725(hereinafter, referred to as ‘non-patent document’), there is providedan amorphous solid electrolyte such as 70Li₂S-30P₂S₅ and 80Li₂S-20P₂S₅among the sulfide-based solid electrolytes. According to the non-patentdocument, the amorphous solid electrolyte has ion conductivity at 1×10⁻³S/cm at the time of heat-treating (crystallizing) at a relatively lowtemperature of 200° C. to 300° C., whereas the amorphous solidelectrolyte has ion conductivity at 1×10⁻⁶ S/cm at the temperature orhigher.

In Korean Patent Application Publication No. 10-2015-0132265(hereinafter, referred to as ‘patent document’), there is provided acrystalline solid electrolyte such as Li₆PS₅Cl among sulfide-based solidelectrolytes. According to the patent document, the crystalline solidelectrolyte has ion conductivity at 1×10⁻³ S/cm at the time ofheat-treating (crystallizing) at a temperature of about 500° C., whereasthe crystalline solid electrolyte has ion conductivity at 1×10⁻⁴ S/cm atthe temperature or less.

As described in non-patent documents and the patent document, there is alimitation that the sulfide-based solid electrolyte in the related arthas high ion conductivity only in a predetermined temperature range ofeither a low temperature (about 250° C.) or a high temperature (about500° C.).

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY

Embodiments of the present disclosure relates to a sulfide-based solidelectrolyte having high ion conductivity in a wide crystallizationtemperature range, as a sulfide-based solid electrolyte containing anickel (Ni) element and a halogen element.

Embodiments of the present invention can overcome the limitation of asulfide-based solid electrolyte in the related art and provide asulfide-based solid electrolyte having high ion conductivity in a widecrystallization temperature range and a preparing method thereof.

The present invention may include a number configurations in order toachieve advantages, for example, solving above-described problemsassociated with the prior art.

In one aspect, the present invention provides a sulfide-based solidelectrolyte including 5 parts by mole to 20 parts by mole of nickelsulfide (Ni₃S₂) and 5 parts by mole to 40 parts by mole of lithiumhalide with respect to 100 parts by mole of a mixture of lithium sulfide(Li₂S) and diphosphorus pentasulfide (P₂S₅).

In a preferred embodiment, the mixture may include 60 mole % to 90 mole% of lithium sulfide and 10 mole % to 40 mole % of diphosphoruspentasulfide.

In another preferred embodiment, the lithium halide may be expressed byLiX (X is Cl, Br or I).

In still another preferred embodiment, the crystallization temperaturemay be 200° C. to 400° C. and the sulfide-based solid electrolyte mayhave a cubic crystal structure.

In yet another preferred embodiment, the crystallization temperature maybe 400° C. to 600° C. and the sulfide-based solid electrolyte may have acubic crystal structure.

In still yet another preferred embodiment, the cubic crystal structuremay have diffraction peaks in an a area of diffraction angles 2θ of15.5±0.5°, 18±0.5°, 25.5±0.5°, 30±0.5°, 31.5±0.5°, 40±0.5°, 45.5±0.5°,48±0.5°, 53±0.5°, 55±0.5°, 56.5±0.5° and 59.5±0.5° in an X-raydiffraction spectrum.

In another aspect, the present invention provides a method of preparinga sulfide-based solid electrolyte including: (1) preparing a startingmaterial by adding 5 parts by mole to 20 parts by mole of nickel sulfide(Ni₃S₂) and 5 parts by mole to 40 parts by mole of lithium halide withrespect to 100 parts by mole of a mixture of lithium sulfide (Li₂S) anddiphosphorus pentasulfide (P₂S₅); (2) milling and amorphizing thestarting material; and (3) heat-treating and crystallizing theamorphized starting material.

In a preferred embodiment, the mixture may include 60 mole % to 90 mole% of lithium sulfide and 10 mole % to a 40 mole % of diphosphoruspentasulfide.

In another preferred embodiment, the lithium halide may be expressed byLiX (X is Cl, Br or I).

In still another preferred embodiment, step (3) may be a step ofheat-treating the amorphized starting material at 200° C. to 400° C. andcrystallizing the heat-treated starting material to have a cubic crystalstructure.

In yet another preferred embodiment, step (3) may be a step ofheat-treating the amorphized starting material at 400° C. to 600° C. andcrystallizing the heat-treated starting material to have a cubic crystalstructure.

In still yet another preferred embodiment, the cubic crystal structuremay have diffraction peaks in an area of diffraction angles 2θ of15.5±0.5°, 18±0.5°, 25.5±0.5°, 30±0.5°, 31.5±0.5°, 40±0.5°, 45.5±0.5°,48±0.5°, 53±0.5°, 55±0.5°, 56.5±0.5° and 59.5±0.5° in an X-raydiffraction spectrum.

The present invention includes the above configurations and thus canhave the following effects.

According to embodiments of the present invention, the sulfide-basedsolid electrolyte can have high ion conductivity due to a crystalstructure of a high ionic conductive phase in a wide temperature rangeeven though a battery is driven in any environment.

Since the range of the crystallization temperature is wide, thesulfide-based solid electrolyte may be used suitably for changes in amanufacturing process and thus commercialization of the sulfide-basedsolid electrolyte can be accelerated.

The effects of the present invention are not limited to theaforementioned effects. It should be understood that the effects of thepresent invention include all effects inferable from the descriptionbelow.

Other aspects and preferred embodiments of the invention are discussedinfra.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated in the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a result of measuring impedance values for sulfide-based solidelectrolytes in Examples 1 to 4 of the present invention;

FIG. 2 is a result of measuring impedance values for sulfide-based solidelectrolytes in Examples 5 to 8 of the present invention;

FIG. 3 is an X-ray diffraction spectroscopy (XRD) result for thesulfide-based solid electrolyte in Example 4 of the present invention;

FIG. 4 is an XRD result for the sulfide-based solid electrolyte inExample 8 of the present invention;

FIG. 5 is a result of measuring discharge capacities of all solid-statebatteries adopting the sulfide-based solid electrolytes in Examples 1 to4 of the present invention; and

FIG. 6 is a result of measuring discharge capacities of all solid-statebatteries adopting the sulfide-based solid electrolytes in Examples 5 to8 of the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

Hereinafter, the present invention will be described in detail throughexemplary embodiments. The exemplary embodiments of the presentinvention may be modified in various forms as long as the gist of theinvention is not changed. However, the scope of the present invention isnot limited to the following exemplary embodiments.

When it is determined that the description for the known configurationsand functions may obscure the gist of the present invention, thedescription for the known configurations and functions will be omitted.In this specification, the term “comprise” means that other constituentelements may be further included unless noted otherwise.

The present invention is a sulfide-based solid electrolyte includinglithium sulfide (Li₂S), diphosphorus pentasulfide (P₂S₅), nickel sulfide(Ni₃S₂) and lithium halide (LiX) and characterized by forming a cubiccrystal structure having high ion conductivity in a wide temperaturerange compared to a sulfide-based solid electrolyte in the related artat the time of heat-treating for crystallization.

A method of preparing a sulfide-based solid electrolyte according to thepresent invention includes (1) a step of preparing a starting materialby adding 5 parts by mole to 20 parts by mole of nickel sulfide (Ni₃S₂)and 5 parts by mole to 40 parts by mole of lithium halide with respectto 100 parts by mole of a mixture of lithium sulfide (Li₂S) anddiphosphorus pentasulfide (P₂S₅); (2) a step of milling and amorphizingthe starting material; and (3) a step of heat-treating and crystallizingthe amorphized starting material.

The lithium sulfide, diphosphorus pentasulfide, nickel sulfide andlithium halide are not particularly limited and may be industriallyavailable or synthesized by a conventional method, and may use materialshaving high purity.

The mixture may include lithium sulfide of 60 mole % to 90 mole %,particularly 70 mole % to 80 mole %, and more particularly 75 mole % to80 mole %, and diphosphorus pentasulfide of 10 mole % to 40 mole %,particularly 20 mole % to 30 mole %, and more particularly 20 mole % to25 mole %.

According to the present invention, the sulfide-based solid electrolytecontaining a nickel (Ni) element may be prepared by adding nickelsulfide to the mixture of lithium sulfide and diphosphorus pentasulfidein (1) the preparing of a starting material.

The sulfide-based solid electrolyte containing the nickel (Ni) elementhas high ion conductivity due to a crystal structure formed with nickel(Ni). The sulfide-based solid electrolyte has a specific crystalstructure according to binding of respective elements and it isestimated that the lithium ions move by a hopping method through a gapin the crystal structure. Accordingly, as van der Waals radius of theelement forming the gap in the crystal structure is decreased, it isadvantageous in movement of lithium ions. The van der Waals radii of theelements mainly included in the sulfide-based solid electrolyte in therelated art are as follows.

-   -   Phosphorus (180 pm), sulfur (180 pm), tin (217 pm), silicon (210        pm), arsenic (185 pm)

Meanwhile, since the van der Waals radius of nickel is 163 pm and verysmall compared to the above elements, when nickel is included in thecrystal structure, the lithium ions may smoothly pass through the gap.

The sulfide-based solid electrolyte containing the nickel (Ni) elementhas excellent stability. This may be described according to a principleof hard and soft acids and bases (HSAB). Since sulfur (S) is a weak baseand phosphorus (P) is a strong acid, sulfur (S) and phosphorus (P) arenot stably bound to each other. As a result, when nickel (Ni) which isan intermediate acid with weaker acidity than phosphorus (P) is includedin a crystal phase, nickel (Ni) has better reactivity than phosphorus(P) as a weak base and high stability during binding.

The nickel sulfide may be added with 5 parts by mole to 20 parts bymole, preferably 5 parts by mole to 15 parts by mole, and morepreferably 5 parts by mole to 10 parts by mole with respect to 100 partsby mole of the mixture of lithium sulfide and diphosphorus pentasulfide.When the content of nickel sulfide is 5 parts by mole or more, ionconductivity and stability of the sulfide-based solid electrolyte may beimproved as described above. Further, when the content of nickel sulfideis 20 parts by mole or less, nickel sulfide may be evenly distributed inthe mixture and finally, the crystal structure of the sulfide-basedsolid electrolyte may be evenly formed.

According to the present invention, the sulfide-based solid electrolytecontaining a nickel (Ni) element and a halogen element may be preparedby adding nickel sulfide and lithium halide to the mixture of lithiumsulfide and diphosphorus pentasulfide in (1) the preparing of a startingmaterial.

The lithium halide may be expressed by LiX (X is Cl, Br or I).

The lithium halide may be added with 5 parts by mole to 40 parts by molewith respect to 100 parts by mole of the mixture of lithium sulfide anddiphosphorus pentasulfide. When the content of lithium halide is 5 partsby mole or more, a cubic crystal structure of a high ion conductivephase may be formed. However, when the content of lithium halide is morethan 40 parts by mole, an orthorhombic crystal structure in which ionconductivity of the sulfide-based solid electrolyte is reduced at thetime of hot heat treatment may be formed.

As such, a precursor (starting material) of the sulfide-based solidelectrolyte is formed by adding the specific content of nickel sulfideand lithium halide to the mixture of lithium sulfide and diphosphoruspentasulfide to form a cubic crystal structure having high ionconductivity in a wide temperature range of 200° C. to 600° C. comparedwith the sulfide-based solid electrolyte in the related art at the timeof heat treatment for crystallization of the starting material.

Step (2) is a step of milling and amorphizing the starting material.

The amorphizing may be performed by a method of wet milling of adding asolvent to the starting material and then milling the solvent or drymilling of milling the starting material without adding the solvent.

When the amorphizing is performed by wet milling, before step (3) to bedescribed below, drying for removing the solvent may be furtherperformed.

Step (3) is a step of heat-treating and crystallizing the amorphizedstarting material to have a specific crystal structure.

It is described earlier that the sulfide-based solid electrolyte in therelated art is formed as a high ion conductive phase only in a specifictemperature range of a low temperature of about 250° C. or a hightemperature of about 500° C.

The sulfide-based solid electrolyte according to the present inventionhas a wide crystallization temperature to have a cubic crystal structureas a high ion conductive phase in the entire numerical range, at thetime of heat treatment at 200° C. to 600° C.

Hereinafter, the present invention will be described in more detailthrough detailed Examples. However, these Examples are to exemplify thepresent invention and the scope of the present invention is not limitedthereto.

EXAMPLES

The following examples illustrate the invention and are not intended tolimit the same.

Comparative Example 1

After 80Li₂S-20P₂S₅ as a sulfide-based solid electrolyte disclosed inMizuno et al., high lithium ion conducting glass-ceramics in the systemLi₂S—P₂S₅ , Solid State Ionics, 177(2006), 2721-2725 was prepared byvarying a crystallization temperature, ion conductivity was measured.The preparing method was as follows.

Li₂S:P₂S₅ was measured to be in a composition ratio of 80 mol:20 mol soas to prepare a mixture of 20 g. Xylene as a solvent was added to themixture and then the mixture was milled by a planetary mill andamorphized. After the amorphizing is completed, the solvent was removedby vacuum drying and the mixture was crystallized by heat-treating undera condition of Table 1 below.

Each sulfide-based solid electrolyte was compressively molded by varyinga heat-treatment temperature to form a molding body (a diameter of 13mm) for measurement. An AC potential of 10 mV was applied to the moldingbody and then ion conductivity was measured by performing a frequencysweeping of 1×10⁶ to 100 Hz. The result is illustrated in Table 1.

TABLE 1 Heat-treatment Ion conductivity Classification (crystallization)condition [S/cm] 80Li₂S—20P₂S₅ 230° C., 2 hrs 1.3 × 10⁻³ 500° C., 4 hrs2.3 × 10⁻⁶

Referring to Table 1, in the case of a sulfide-based solid electrolytein Comparative Example 1, it can be seen that high ion conductivity of10⁻³ S/cm or more is observed at a low crystallization temperature of230° C., whereas ion conductivity is very low at a high crystallizationtemperature of 500° C.

Comparative Example 2

Li₆PS₅Cl as a sulfide-based solid electrolyte disclosed in Korean PatentApplication Publication No. 10-2015-0132265 was prepared by varying acrystallization temperature and then ion conductivity was measured. Thepreparing method was as follows.

A lithium sulfide powder, a diphosphorus pentasulfide powder, and alithium chloride (LiCl) powder were measured according to a compositionof Li₆PS₅Cl to prepare a mixture of 5 g. Xylene as a solvent was addedto the mixture, and then the mixture was milled by a planetary mill andamorphized. After the amorphizing is completed, the solvent was removedby vacuum drying and the mixture was crystallized by heat-treating undera condition of Table 2 below.

Ion conductivity was measured by the same method as ComparativeExample 1. The result is illustrated in Table 2 below.

TABLE 2 Heat-treatment Ion conductivity Classification (crystallization)condition [S/cm] Li₆PS₅Cl 230° C., 2 hrs 4.31 × 10⁻⁴ 500° C., 4 hrs 1.81× 10⁻³

Referring to Table 2, in the case of a sulfide-based solid electrolytein Comparative Example 2, it can be seen that high ion conductivity of10⁻³ S/cm or more is observed at a high crystallization temperature of500° C., whereas ion conductivity is slightly low at a lowcrystallization temperature of 230° C.

Examples 1 to 4 Example 1

6.85 g of lithium sulfide (Li₂S), 8.28 g of diphosphorus pentasulfide(P₂S₅), 4.48 g of nickel sulfide (Ni₃S₂), and 0.39 g of lithium chloride(LiCl) were measured and mixed to be Chemical Formula of Table 3 toprepare 20 g of a starting material.

To the starting material, 165 g of xylene as a solvent was added andthen milled by a planetary ball mill and amorphized. Thereafter, thesolvent was removed by vacuum drying under conditions of about 160° C.and 2 hrs.

The amorphized starting material was crystallized by heat-treating underconditions of about 260° C. and 2 hrs to obtain the sulfide-based solidelectrolyte.

Example 2

Except that 6.72 g of lithium sulfide (Li₂S), 8.12 g of diphosphoruspentasulfide (P₂S₅), 4.39 g of nickel sulfide (Ni₃S₂), and 0.77 g oflithium chloride (LiCl) were measured and mixed to be Chemical Formulaof Table 3 to prepare 20 g of a starting material, a sulfide-based solidelectrolyte was obtained by the same method as Example 1.

Example 3

Except that 6.47 g of lithium sulfide (Li₂S), 7.82 g of diphosphoruspentasulfide (P₂S₅), 4.22 g of nickel sulfide (Ni₃S₂), and 1.49 g oflithium chloride (LiCl) were measured and mixed to be Chemical Formulaof Table 3 to prepare 20 g of a starting material, a sulfide-based solidelectrolyte was obtained by the same method as Example 1.

Example 4

Except that 6.02 g of lithium sulfide (Li₂S), 7.28 g of diphosphoruspentasulfide (P₂S₅), 3.93 g of nickel sulfide (Ni₃S₂), and 2.77 g oflithium chloride (LiCl) were measured and mixed to be Chemical Formulaof Table 3 to prepare 20 g of a starting material, a sulfide-based solidelectrolyte was obtained by the same method as Example 1.

Examples 5 to 8 Example 5

Except for heat-treating and crystallizing the amorphized startingmaterial under conditions of 500° C. and 4 hrs, a sulfide-based solidelectrolyte was obtained by the same method as Example 1.

Example 6

Except for heat-treating and crystallizing the amorphized startingmaterial under conditions of 500° C. and 4 hrs, a sulfide-based solidelectrolyte was obtained by the same method as Example 2.

Example 7

Except for heat-treating and crystallizing the amorphized startingmaterial under conditions of 500° C. and 4 hrs, a sulfide-based solidelectrolyte was obtained by the same method as Example 3.

Example 8

Except for heat-treating and crystallizing the amorphized startingmaterial under conditions of 500° C. and 4 hrs, a sulfide-based solidelectrolyte was obtained by the same method as Example 4.

Compositions and crystallization conditions in Examples 1 to 8 areillustrated in Table 3 below.

TABLE 3 Crystallization Classification Chemical Formula conditionExample 1 80Li₂S•20P₂S₅•10Ni₃S₂•5LiCl 260° C./2 hrs Example 280Li₂S•20P₂S₅•10Ni₃S₂•10LiCl Example 3 80Li₂S•20P₂S₅•10Ni₃S₂•20LiClExample 4 80Li₂S•20P₂S₅•10Ni₃S₂•40LiCl Example 580Li₂S•20P₂S₅•10Ni₃S₂•5LiCl 500° C./4 hrs Example 680Li₂S•20P₂S₅•10Ni₃S₂•10LiCl Example 7 80Li₂S•20P₂S₅•10Ni₃S₂•20LiClExample 8 80Li₂S•20P₂S₅•10Ni₃S₂•40LiCl

Example 9

Except that 5.25 g of lithium sulfide (Li₂S), 6.35 g of diphosphoruspentasulfide (P₂S₅), 3.43 g of nickel sulfide (Ni₃S₂), and 4.96 g oflithium bromide (LiBr) were measured and mixed to be Chemical Formula of80Li₂S.20P₂S₅.10Ni₃S₂.40LiBr to prepare 20 g of a starting material, asulfide-based solid electrolyte was obtained by the same method asExample 1.

Example 10

Except that 4.63 g of lithium sulfide (Li₂S), 5.60 g of diphosphoruspentasulfide (P₂S₅), 3.03 g of nickel sulfide (Ni₃S₂), and 6.74 g oflithium iodide (LiI) were measured and mixed to be Chemical Formula of80Li₂S.20P₂S₅.10Ni₃S₂.40LiI to prepare 20 g of a starting material, asulfide-based solid electrolyte was obtained by the same method asExample 1.

TEST EXAMPLES Test Example 1

Ion conductivities of the sulfide-based solid electrolytes in Examples 1to 8 were measured. Each sulfide-based solid electrolyte wascompressively molded to form a molding body (a diameter of 13 mm) formeasurement. An AC potential of 10 mV was applied to the molding bodyand then an impedance value was measured by performing a frequencysweeping of 1×10⁶ to 100 Hz to obtain ion conductivity.

FIG. 1 is a result of measuring impedance values for the sulfide-basedsolid electrolytes in Examples 1 to 4 of the present invention.

FIG. 2 is a result of measuring impedance values for the sulfide-basedsolid electrolytes in Examples 5 to 8 of the present invention.

Table 4 below is a result of measuring ion conductivities for thesulfide-based solid electrolytes in Examples 1 to 8 of the presentinvention.

TABLE 4 Classification Ion conductivity [S/cm] Example 1 7.00 × 10⁻⁴Example 2 1.03 × 10⁻³ Example 3 1.06 × 10⁻³ Example 4 1.09 × 10⁻³Example 5 2.33 × 10⁻⁴ Example 6 2.93 × 10⁻⁴ Example 7 6.88 × 10⁻⁴Example 8 2.07 × 10⁻³

Referring to FIGS. 1 and 2, it can be seen that ion conductivity isincreased as the content of halogen element in the sulfide-based solidelectrolyte is increased. Further, referring to FIG. 2, in the case ofcrystallization at a high temperature of 500° C., it can be verifiedthat an interface resistance is decreased as the content of halogenelement is increased.

Referring to Table 4, it can be seen that the sulfide-based solidelectrolyte according to the present invention has ion conductivity of10⁻⁴ S/cm or more in a wide crystallization temperature range.Particularly, it can be verified that Examples 4 and 8 containing 40parts by mole of lithium halide have very high ion conductivity of 10⁻³S/cm or more.

Test Example 2

With respect to the sulfide-based solid electrolytes in Examples 4 and8, an X-ray diffraction spectroscopy (XRD) was performed.

FIG. 3 is an XRD result for the sulfide-based solid electrolyte inExample 4 and FIG. 4 is an XRD result for the sulfide-based solidelectrolyte in Example 8.

Referring to FIG. 3, it can be seen that the sulfide-based solidelectrolyte in Example 4 has diffraction peaks in regions havingdiffraction angles 2θ of 15.5±0.5°, 18±0.5°, 25.5±0.5°, 30±0.5°,31.5±0.5°, 40±0.5°, 45.5±⁰0.5°, 48±0.5°, 53±0.5°, 55±0.5°, 56.5±0.5° and59.5±0.5° as main peaks, and it can be seen that since the peaks aresubstantially the same peaks as Li₆PS₅Cl having high ion conductivity,the sulfide-based solid electrolyte of Example 4 has a high ionconductive cubic crystal structure.

Referring to FIG. 4, it can be seen that since the sulfide-based solidelectrolyte in Example 8 also has substantially the same peaks asLi₆PS₅Cl as main peaks, the sulfide-based solid electrolyte of Example 8has a high ion conductive cubic crystal structure.

As a result, it can be seen that the sulfide-based solid electrolyteaccording to the present invention has a high ion conductive cubiccrystal structure in a wide crystallization temperature range and itsupports the result of Test Example 1.

Test Example 3

Charge and discharge capacities of an all solid-state batteries adoptingthe sulfide-based solid electrolytes in Examples 1 to 8 were measured.

The all solid-state battery was constituted by a positive electrode, anegative electrode, and a solid electrolyte layer interposed between thepositive electrode and the negative electrode. The solid electrolytelayer was formed with a thickness of 500 μm by compressively molding thesulfide-based solid electrolytes in Examples 1 to 8, and as the positiveelectrode, a powder containing an active material (Nb-coated NCM622), asolid electrolyte (a solid electrolyte used in the solid electrolytelayer), and a conductive material (Super C) was formed on the solidelectrolyte layer with an active material loading amount of 50.8 mg/cm²and a thickness of 30 μm, and as the negative electrode, indium foilwith a thickness of 100 μm was used.

With respect to the all solid-state battery, a discharge capacity wasmeasured by performing charging and discharging under a constant current(CC) condition in a range of 2 V to 3.58 V at rate limiting of 0.02 Crate.

FIG. 5 is a result of measuring charge and discharge capacities of allsolid-state batteries adopting the sulfide-based solid electrolytes inExamples 1 to 4 of the present invention and FIG. 6 is a result ofmeasuring charge and discharge capacities of all solid-state batteriesadopting the sulfide-based solid electrolytes in Examples 5 to 8 of thepresent invention. The charge and discharge capacities are numericallyillustrated in Table 5.

TABLE 5 Classification Charge [mAh/g] Discharge [mAh/g] Example 1 185137 Example 2 166 115 Example 3 153 109 Example 4 182 143 Example 5 197131 Example 6 221 147 Example 7 250 149 Example 8 218 150

Referring to FIGS. 5 and 6 and Table 5, it can be verified that the allsolid-state batteries adopting the sulfide-based solid electrolytes inExamples 1 to 8 have excellent charge and discharge capacities of about150 mAh/g.

Test Example 4

Ion conductivities for the sulfide-based solid electrolytes in Examples9 and 10 were measured by the same method as Test Example 1. The resultis illustrated in Table 6.

TABLE 6 Ion conductivity Classification Chemical Formula [S/cm] Example4 80Li₂S•20P₂S₅•10Ni₃S₂•40LiCl 1.09 × 10⁻³ Example 980Li₂S•20P₂S₅•10Ni₃S₂•40LiBr 5.10 × 10⁻⁴ Example 1080Li₂S•20P₂S₅•10Ni₃S₂•40LiI 3.00 × 10⁻⁴

Referring to Table 6, it can be seen that even in the case of usinglithium bromide and lithium iodide instead of lithium chloride as thelithium halide, the sulfide-based solid electrolyte having high ionconductivity which has excellent ion conductivity of 1.0×10⁻⁴ S/cm ormore can be synthesized.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

What is claimed is:
 1. A method of preparing a sulfide-based solidelectrolyte, the method comprising: preparing a starting material byadding 5 parts by mole to 20 parts by mole of nickel sulfide (Ni₃S₂) and5 parts by mole to 40 parts by mole of lithium halide with respect to100 parts by mole of a mixture of lithium sulfide (Li₂S) anddiphosphorus pentasulfide (P₂S₅); milling the starting material toobtain an amorphous material; and heat-treating the amorphous materialat a temperature range of 200° C. to 260° C. or 400° C. to 600° C.,wherein a crystallized sulfide-based solid electrolyte is obtained afterthe heat-treating, wherein the heat-treated sulfide-based solidelectrolyte has a cubic crystal structure that has diffraction peaks inan area of diffraction angles 2θ of 15.5±0.5°, 18±0.5°, 25.5±0.5°,30±0.5°, 31.5±0.5°, 40±0.5°, 45.5±0.5°, 48±0.5°, 53±0.5°, 55±0.5°,56.5±0.5° and 59.5±0.5° in an X-ray diffraction spectrum.
 2. The methodof claim 1, wherein the mixture comprises 60 mole % to 90 mole % of thelithium sulfide; and 10 mole % to 40 mole % of diphosphoruspentasulfide.
 3. The method of claim 1, wherein the lithium halide isexpressed by LiX, where X is Cl, Br or I.
 4. The method of claim 1,wherein the heat-treating is performed at 400° C. to 600° C.
 5. Themethod of claim 1, wherein, the heat-treating is performed at 200° C. to260° C.
 6. The method of claim 3, wherein X is Cl.
 7. The method ofclaim 3, wherein X is Br.
 8. The method of claim 3, wherein X is I. 9.The method of claim 1, wherein the method forms a sulfide-based solidelectrolyte having a cubic crystal structure and comprising: withrespect to 100 parts by mole of a mixture of lithium sulfide (Li₂S) anddiphosphorus pentasulfide (P₂S₅); 5 parts by mole to 20 parts by mole ofnickel sulfide (Ni₃S₂); and 5 parts by mole to 40 parts by mole oflithium halide.
 10. The method of claim 9, wherein the mixture comprises60 mole % to 90 mole % of the lithium sulfide; and 10 mole % to 40 mole% of diphosphorus pentasulfide.
 11. The method of claim 9, wherein thelithium halide is expressed by LiX, where X is Cl, Br or I.
 12. Themethod of claim 9, wherein a crystallization temperature of thesulfide-based solid electrolyte is 200° C. to 260° C.
 13. The method ofclaim 9, wherein a crystallization temperature of the sulfide-basedsolid electrolyte is 400° C. to 600° C.
 14. The method of claim 9,wherein the sulfide-based solid electrolyte has high ion conductivity ina wide crystallization temperature range.